Exhaust-gas sampling systems and the appertaining methods for their operation have previously been described and are subject to legal regulations to the effect that the motors of automobiles are not allowed to exceed specific emission limits, such as, for example, the ECE Guideline R 83 for the territory of Europe or the Code of Federal Regulations No. 40 for the United States. These regulations for the most part govern not only the emission limits, but also the type of sampling by systems with variable dilution for emission measurement.
Systems of the above type are known, for example, as CVS systems (constant volume sampling). In these systems, the exhaust gas is admixed with a quantity of air to generate a largely constant total volume flow of the air/exhaust mixture. The samples removed from such systems in bags are then analyzed for their emission contents with the aid of an analyzer. There will in particular be measured carbon dioxide, carbon monoxide, hydrocarbon, and nitrogen oxide contents.
A residue of the sample gas will normally remain in the sample bags after analysis. Before starting a new drive cycle, it is thus required that the sample bags be completely evacuated. This is usually performed by a vacuum pump which is arranged in a separate evacuation line leading to an outlet.
A device of the above type is described, for example, in DE 198 57 955 A1 where the sample bags are arranged within a pressure-tight container adapted to be evacuated via a vacuum conveying means. In the evacuation line as well as in the main conveying lines which lead to the sample bags, and in the lines leading to the analyzer, respective valves are arranged by which the lines can be closed or cleared. The container is subjected to a constant vacuum during the filling of the bags, thus avoiding condensation of the water vapor existent in the exhaust gas. By opening the outlet valves of the sample bags (after the measurement has been performed), the sample bags will also be completely evacuated by the vacuum conveying means.
For space- and cost-saving reasons, the vacuum conveying means is realized in the form of relatively small pumps with small conveying capacities. This has the consequence that the time for evacuation, and thus the time between two to-be-measured drive cycles, is relatively long.